1. Field of the Invention
The present invention relates to a light emitting diode device used for a copying machine, a printer, a display, an original illuminating light source of an image reading apparatus, and the like, and a method of manufacturing the light emitting diode device.
2. Description of the Related Art
A light emitting diode device, in which a plurality of pixels each comprising a micro light emitting portion are arranged to permit the emission of light from each of the pixels to be independently controlled, is conventionally used as a panel or a back light of a display device or the like; an internal writing head of an image forming apparatus such as a copying machine, a printer, or the like; or an original illuminating light source of an image reading apparatus.
Such a conventional light emitting diode device is produced by a method comprising arranging light emitting diode chips comprising an inorganic semiconductor crystal such as GaAs or AlInGaP in a one-dimensional or two-dimensional form, or a method comprising depositing a thin film of an inorganic material such as ZnS or the like on a substrate, and then patterning the thin film to form pixels.
An example of a display device using light emitting diode chips which is brought into practical use is a large outdoor display device of a several meters square in which packages of respective light emitting diode chips are arranged in a two-dimensional form.
Also an internal reading head of an image forming apparatus such as a copying machine, a printer, or the like is brought into practical use, in which light emitting chips each having many micro light emitting portions provided thereon are arranged in a one-dimensional form.
However, the method of arranging light emitting chips has the problem of causing difficulties in miniaturizing the apparatus and improving arrangement precision.
The method of depositing a thin film of an inorganic material such as ZnS or the like on a substrate, and then patterning the thin film to form pixels is suitable for miniaturizing the apparatus, and permits photolithographic patterning or the like with high precision. This method does not have the above-described problem, but has a problem in which the device cannot be driven with a direct current at a low voltage suitable for design of an electronic apparatus.
On the other hand, an organic thin film light emitting diode (organic light emitting element) has recently been developed, which can be formed in a thin film on a substrate of a large area, and which can be driven with a direct current. In order to solve the above problems, therefore, the use of an organic light emitting device as a light emitting device is proposed, in which light emitting portions comprising organic light emitting elements are arranged in a one-dimensional or two-dimensional form.
FIG. 70 shows a typical structure of an organic light emitting diode.
Referring to FIG. 70, the organic light emitting diode comprises a substrate 100, a transparent electrode 401 made of indium/tin oxide (ITO), a hole transport layer 403 made of an organic hole transport material such as an aromatic diamine (Formula 2) or the like, and an organic electron transport layer 404 made of an organic electron transport material such as tris(8-quinolinolato)aluminum complex (Formula 3).
The organic light emitting diode further comprises a cathode made of a material having a low work function, such as Al, a Mg:Ag alloy, or the like. When a voltage is applied between an anode and cathode, holes injected into the hole transport layer from the anode are recombined with electrons injected into the electron transport layer from the cathode through an electron injection layer to emit light.
As the simplest method for arranging a plurality of micro pixels each comprising such a light emitting diode to enable independent control of the emission of light from each of the pixels, a plurality of parallel stripe anodes are formed on a substrate, the hole transport layer and the electron transport layer are laminated on the anode, and stripe cathodes are further formed on the hole and electron transport layers perpendicularly to the anodes to form a simple matrix in which a pixel is formed at each of the intersections of the anodes and the cathodes. In order to drive this structure, for example, the lines of the cathodes formed in parallel stripes are successively connected to a negative power source and disconnected therefrom, and in synchronism therewith, the anodes are successively connected to a positive power source or disconnected therefrom. By this operation, each of the pixels is flashed only at a moment when the cathode connected to the corresponding pixel is connected to the negative power source depending upon whether or not the anode is connected to the positive power source at that moment.
In this simple method, since only one of the plurality of cathode lines is connected to the negative power source at a moment, only the pixels connected to the line connected to the negative power source are flashed depending upon whether or not the anodes are connected to the positive power source, all other pixels being turned off regardless of whether the anodes are connected to the positive power source. This operation has the property that the lighting duty of the pixels decreases with an increase in the number of lines of the cathodes.
Therefore, this method has the drawback that although the luminance is high at a lighting moment, the effective luminance as an average luminance for a predetermined time decreases as the number of pixels increases to increase the number of cathode lines.
In order to improve this point, a light emitting diode device has been proposed in which a non-linear element such as a transistor or capacitor is provided on each of the pixels.
Of devices using the above-mentioned organic light emitting diode, an example of light emitting diode devices in which light emitting portions used in a display device are arranged in a two-dimensional form is described below.
FIG. 71 is a drawing showing an equivalent circuit of a single pixel in such a light emitting diode device.
The circuit shown in FIG. 71 comprises a first thin film transistor (address transistor) 1 which constitutes a pixel, a storage capacitor 2, a second thin film transistor (driving transistor), and an organic light emitting diode 4. The circuit further comprises an electrode Ps connected to the source electrode of the address transistor, an electrode Pm connected to the second side of the storage capacitor and the gate electrode of the driving transistor, an electrode Pg connected to the gate electrode of the address transistor, an electrode Pc connected to the first side of the storage capacitor and the source electrode of the driving transistor, and an electrode Pled connected to a cathode of the organic light emitting diode 4.
A selection signal is applied to the electrode Pg, a data signal is applied to the electrode Ps, and a potential appears in the electrode Pm by charge and discharge of the storage capacitor according to the data signal. The electrodes Pc and Pled are at fixed potentials.
The circuit is operated as described below.
When the selection signal applied to the electrode Pg is brought to a selection state (high-potential state), the potential of the electrode Pg decreases. As a result, conduction occurs between the source and drain of the address transistor, and a current flows in and out of the storage capacitor 2 according to the data signal applied to the electrode Ps to set a potential difference between the source and gate electrodes of the driving transistor, i.e., a potential difference between the electrodes Pc and Pm, to a value corresponding to the data signal applied to the electrode Ps. Therefore, a current flows through the driving transistor 3 according to the data signal, and the organic light emitting diode 4 emits light with a luminance corresponding to the data signal. When the selection signal applied to the electrode Pg is brought to a non-selection state (low-potential state), the source and drain of the address transistor are made nonconductive, and no current flows in and out of the storage capacitor 2 through the address transistor 1 even if the data signal applied to the electrode Pd changes. Therefore, there is little change in the potential difference between the electrodes Pc and Pm, thereby hardly, if at all, affecting the emission of light from the organic light emitting diode 4.
FIG. 72 is a plan view showing four pixels in a portion of such an organic light emitting device.
Each of the pixels comprises a first thin film transistor (address transistor) 1, a second thin film transistor (driving transistor) 3, and a light emitting element 4 which constitutes a light emitting portion.
FIG. 73 is a sectional view of thin film transistors which constitute each of pixels of a conventional organic light emitting diode device.
A driving transistor comprises a substrate 100, a gate electrode 301 made of amorphous silicon, a gate insulating layer 302 made of SiO2, an active layer 303 made of amorphous silicon, and source and drain electrodes 306 made of Al.
In such a light emitting diode device, an address transistor, a driving transistor and a storage capacitor are provided in each of pixels to store charge in the storage capacitor according to a data signal during the selection period, thereby continuously emitting light from the light emitting element during the non-selection period according to the stored charge. Therefore, even in a device in which many pixels are arranged, the light emission duty of each pixel is high, and the effective luminance does not deteriorate.
However, in a conventional light emitting diode device, since a transistor of each of the pixels is made of polycrystalline silicon or amorphous silicon, after the process for forming a transistor made of polycrystalline silicon or amorphous silicon on a substrate, the process for forming an organic light emitting element must be further carried out. For example, a labor-intensive and time-consuming process is required, in which an amorphous silicon film is deposited by a plasma CVD apparatus and then scanned by a laser beam and annealed to be converted to a polycrystalline silicon film. This causes an increase in cost.
On the other hand, Japanese Patent Laid-Open No. 7-57871 discloses an active matrix field light emitting display device comprising a three-terminal field light emitting element and a thin film transistor, both of which are provided in each of pixels, wherein the thin film transistor is made of an organic semiconductor thin film. The use of the organic semiconductor thin film for the thin film transistor has the advantage that it is unnecessary to deposit an a-Si film and form an island.
However, the light emitting display device disclosed in this publication uses the three-terminal light emitting elements, and thus the actual manufacturing process, which requires much time, must be taken into consideration because the structure is relatively complicated as compared with a general diode type. Also control means is required for driving the light emitting display device.
The present invention has been achieved in consideration of the above problems, and an object of the present invention is to provide a light emitting diode device in which the emission duty of pixels is high, the time-average effective luminance for a predetermined time is high, and the manufacturing cost is low.
Another object of the present invention is to provide a light emitting diode device having a simple configuration and high controllability.
A further object of the present invention is to provide a light emitting diode device comprising a light emitting diode and a non-linear element, which are arranged on a substrate, wherein the non-linear element is composed of an organic compound layer.
A still further object of the present invention is to provide a method of manufacturing a light emitting diode device comprising a light emitting diode and a non-linear element which are arranged on a substrate, wherein each of the light emitting diode and the non-linear element is composed of an organic compound layer.
The manufacturing method includes a mode in which the organic compound layers which respectively constitute the light emitting diode and the non-linear element comprise different materials.
The manufacturing method includes a mode in which the organic compound layers which respectively constitute the light emitting diode and the non-linear element comprise the same material.
The light emitting diode device and the manufacturing method therefor of the present invention permit the formation of a simple structure and a non-linear element using an organic compound layer. This can simplify the process and decrease the cost, as compared with a conventional method of forming a non-linear element by using amorphous silicon or polycrystalline silicon. In addition, by using, as the non-linear element, an element such as a diode, a MIM element, or the like, in which the resistance of the element is controlled for a current flowing in the thickness direction of the organic layer, it is possible to relax limits to material selection, and obtain an element having well-balanced characteristics and cost.
The light emitting diode device and the manufacturing method therefor of the present invention permits realization of an organic light emitting element having a simple structure, high emission strength, and little cross talk by a simplified process.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.